Conveyance device, printing device, and conveyance method

ABSTRACT

A conveyance device including an upstream roller that supplies a sheet medium to be processed to a conveyance path; a downstream roller that conveys the supplied medium to a processing position; and a control unit that controls driving the upstream roller and the downstream roller to convey the sheet medium at a constant speed. The control unit determines the starting time of the upstream roller and the downstream roller when starting the conveyance operation based on specific information about the upstream roller and the downstream roller in a previous conveyance operation.

BACKGROUND

1. Technical Field

The present invention relates to devices that use two sets of rollers to convey sheet media to a processing position, and relates more particularly to a conveyance device, printing device, and conveyance method that can eliminate the effect of back tension on the downstream rollers without increasing device size.

2. Related Art

A means of conveying paper or other types of sheet media is required in order to process such media in a printer or other device. Such conveyance devices generally have upstream rollers that supply the media from the part storing the sheet medium to the conveyance path, and downstream rollers that convey the supplied medium through the conveyance path to the position where the media is printed or otherwise processed.

Such conveyance devices must be able to accurately control the conveyance speed of the medium from the downstream rollers in order to apply the printing or other process to the conveyed medium with good precision and high quality. Such control is difficult, however, if there is back tension from the upstream side pulling on the medium at the downstream rollers.

Technology for overcoming this problem is taught in Japanese Unexamined Patent Appl. Pub. JP-A-2008-56367. JP-A-2008-56367 teaches advancing the drive time or increasing the paper feed distance of the upstream rollers.

However, consistently controlling media conveyance appropriately as conveyance conditions change is a problem with the technology taught in JP-A-2008-56367 because the drive time is advanced or the paper feed distance is increased in the same way regardless of the current conditions. For example, the force applied to the rollers and the conveyance force of the rollers change according to roller wear and media storage conditions (such as the roll diameter when the medium is roll paper). As a result, if control is always based on the same fixed values, slack in the medium between the upstream rollers and downstream rollers may be lost, or excess slack may allow the medium to rub against parts disposed along the conveyance path, and back tension maybe applied to the downstream rollers. Another problem is that device size must be increased in order to maintain excess slack at all times so that slack is not eliminated and prevent slack media from touching parts along the conveyance path.

SUMMARY

A conveyance device, a printing device, and a media conveyance method according to the present invention enable conveying sheet media using two sets of rollers to a process position while eliminating the effect of back tension on the downstream rollers without increasing device size.

One aspect of the invention is a conveyance device including: an upstream roller that supplies a sheet medium to be processed to a conveyance path; a downstream roller that conveys the supplied medium to a processing position; and a control unit that controls driving the upstream roller and the downstream roller to convey the sheet medium at a constant speed, and determines the starting time of the upstream roller and the downstream roller when starting the conveyance operation based on specific information about the upstream roller and the downstream roller in a previous conveyance operation.

The specific information is information about the behavior of the upstream roller and the downstream roller in the previous conveyance operation until reaching the constant speed, or drive information at the constant speed of the upstream roller and the downstream roller in the previous conveyance operation.

In another aspect of the invention, determination of the starting time preferably delays the starting time of the downstream roller relative to the upstream roller.

In another aspect of the invention, the information about the behavior until reaching the constant speed is the time from when the upstream roller and the downstream roller start until the constant speed is reached.

In another aspect of the invention, the information about the behavior until reaching the constant speed is the amount the sheet medium is respectively conveyed by the upstream roller and the downstream roller from when the upstream roller and the downstream roller start until the time required for the upstream roller to reach the constant speed after starting passes.

In another aspect of the invention, the drive information at the constant speed is information indicating the amount of power required to drive the upstream roller and the downstream roller.

In another aspect of the invention, information indicating the relationship between the information about the behavior until reaching the constant speed or the drive information at the constant speed, and the starting time, is stored in advance for different types of sheet media.

Further preferably in another aspect of the invention, the sheet medium is supplied from a roll to the upstream roller.

Another aspect of the invention is a printing device that has the conveyance device described above, and prints on the sheet medium at the processing position.

Another aspect of the invention is a conveyance method of a conveyance device including an upstream roller that supplies a sheet medium to be processed to a conveyance path, a downstream roller that conveys the supplied medium to a processing position, and a control unit that controls driving the upstream roller and the downstream roller to convey the sheet medium at a constant speed, wherein: the control unit determines the starting time of the upstream roller and the downstream roller when starting the conveyance operation based on specific information about the upstream roller and the downstream roller in a previous conveyance operation.

The specific information is information about the behavior of the upstream roller and the downstream roller in the previous conveyance operation until reaching the constant speed, or drive information at the constant speed of the upstream roller and the downstream roller in the previous conveyance operation.

In another aspect of the invention, the information about the behavior until reaching the constant speed is the time from when the upstream roller and the downstream roller start until the constant speed is reached, or is the amount the sheet medium is respectively conveyed by the upstream roller and the downstream roller from when the upstream roller and the downstream roller start until the time required for the upstream roller to reach the constant speed after starting passes.

In another aspect of the invention, the drive information at the constant speed is information indicating the amount of power required to drive the upstream roller and the downstream roller.

Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a preferred embodiment of a printing device having a conveyance device according to the invention.

FIG. 2 shows an example of the behavior of a supply roller 29 and conveyance roller 30 when the media conveyance operation starts.

FIG. 3 is a flow chart showing steps in the process executed by a conveyance control unit 22.

FIG. 4 describes wait time ΔT.

FIG. 5 shows an example of change in the duty of motors 27A and 27B over time.

DESCRIPTION OF EMBODIMENTS

A preferred embodiment of the invention is described below with reference to the accompanying figures. The following embodiment does not, however, limit the technological scope of the invention. Note that identical or similar parts are notified in the figures by the same reference numerals or reference symbols.

FIG. 1 is a block diagram of a preferred embodiment of a printing device having a conveyance device according to the invention. The printer 2 shown in FIG. 1 is a printing device according to this embodiment of the invention. The printing device conveys paper 26 used as the print medium past a printing position using a supply roller 29 (upstream roller) and conveyance roller 30 (downstream roller), and performs a printing process. The printing device determines when to start driving each of the rollers in each conveyance operation based on the behavior of both rollers when driving started in the previous conveyance operation in order to maintain a constant amount of slack in the paper 26 between the rollers.

As shown in FIG. 1, the printer 2 is a device that receives commands from a computer or other host device 1 and performs a printing process, and in this embodiment is a printing device that uses roll paper 25 as the paper 26 and prints continuously while conveying the paper 26.

FIG. 1 is a block diagram showing the configuration of the printer 2. The printer 2 has a print system that performs a printing process on the paper 26, and a media conveyance system that handles conveying the paper 26.

A print control unit 21 is disposed to the print system. The print control unit 21 receives print commands from the host device 1, and outputs print commands to the printhead unit 23 and paper 26 conveyance commands to the conveyance control unit 22 of the media conveyance system based on the received commands. The printhead unit 23 prints on the paper 26 moving at a specific speed between the printhead unit 23 and platen 24 according to the print commands.

As shown in FIG. 1, the conveyance control unit 22 of the media conveyance system performs a conveyance operation that conveys the paper 26 stored as a paper roll 25 in the print medium storage location through a conveyance path 33 to the printhead unit 23, and then discharges the paper 26 from the printer 2 through the discharge rollers 32.

A supply roller 29 (upstream roller) and conveyance roller 30 (downstream roller) that are driven by respective motors (27A, 27B) are provided for conveying the paper to the printhead unit 23. A follower roller (28A, 28B) is disposed applying pressure to the paper 26 at a position opposite each of these rollers with the paper 26 therebetween.

The supply roller 29 functions to supply the paper 26 held as a paper roll 25 to the conveyance path 33, is driven by torque from the motor 27A transferred through a speed reducer, and conveys the paper 26 by means of friction produced by pressure applied to the paper 26 between the supply roller 29 and follower roller 28A.

The conveyance roller 30 functions to convey the paper 26 supplied by the supply roller 29 to the printing position, or more specifically to the printhead unit 23 position, is driven by torque from the motor 27B transferred through a speed reducer, and conveys the paper 26 by means of friction produced by pressure applied to the paper 26 between the conveyance roller 30 and follower roller 28B.

Encoders 31A and 31B respectively disposed to the supply roller 29 and conveyance roller 30 detect the speed of the corresponding rollers and output the detected speed of each roller to the conveyance control unit 22.

The conveyance control unit 22 shown in FIG. 1 controls the media conveyance system, and controls the conveyance operation of the paper 26 based on commands from the print control unit 21. More specifically, the conveyance control unit 22 controls driving and stopping the supply roller 29 and conveyance roller 30 to desirably convey the paper 26 to the printing position. The method of controlling driving and stopping the supply roller 29 and conveyance roller 30 is a characteristic feature of this printer 2, and is described in detail below.

The conveyance control unit 22 includes a CPU, ROM, RAM and NVRAM (nonvolatile memory) not shown in the figures, and the foregoing process performed by the conveyance control unit 22 is achieved by the CPU operating primarily according to a program stored in ROM.

Required process data is temporarily stored in RAM, and the wait time ΔT described below and drive data from the conveyance operation required for controlling driving and stopping the supply roller 29 and conveyance roller 30 are also stored in RAM. The stored drive data includes the drive start time and conveyance speed of the supply roller 29 and conveyance roller 30, and the duty of each corresponding motor 27 (the current supplied to the motors 27 in this example), are included in the stored drive data.

Relationship information for determining the wait time ΔT is stored in advance in NVRAM. This relationship information is described below.

Note that the media conveyance system including the supply roller 29, conveyance roller 30, and conveyance control unit 22 corresponds to the conveyance device of the invention.

The printer 2 configured as described above is characterized by the method of controlling paper 26 conveyance as described in detail below.

As described above, the printer 2 performs a printing process on the paper 26 conveyed at a specific (constant) speed. The conveyance control unit 22 controls driving the supply roller 29 and conveyance roller 30 so that the conveyance speed of the rollers quickly reaches the specific speed when the printing process starts, maintains that conveyance speed until the printing process ends, and then stops both rollers when the printing process ends. This conveyance operation and conveyance process are repeated each time the printing process is executed.

When the paper 26 is first loaded, the conveyance control unit 22 controls the rollers so that a specific amount of slack (slack such as shown in FIG. 1, for example) is created in the paper 26 between the supply roller 29 and conveyance roller 30, and conveys the paper 26 to a specific position. As described above, this is to prevent back tension from acting on the conveyance roller 30, and thereby enables consistently supplying the paper 26 at a constant speed from the conveyance roller 30 to the printing position.

In addition, because the supply roller 29 receives the force for pulling the paper 26 from the paper roll 25 located upstream as back tension, the supply roller 29 is normally subject to greater back tension than the conveyance roller 30 during paper 26 conveyance.

The supply roller 29 therefore tends to take more time to reach the specific speed at the start of the conveyance operation. FIG. 2 shows an example of the behavior of the supply roller 29 and conveyance roller 30 at the start of the conveyance operation. The x-axis in FIG. 2 shows the time (T) passed after drive starts, and the y-axis shows the conveyance speed (V) of each roller. Curve A in the graph shows the behavior of the supply roller 29, and curve B shows the behavior of the conveyance roller 30.

As described above, because greater back tension is applied to the supply roller 29 than the conveyance roller 30, the rise of the conveyance speed to the targeted specific speed is more gradual for the supply roller 29 (curve A) as shown in FIG. 2. A difference in the amount of paper advanced by both rollers therefore occurs before both rollers reach the specified speed. A conveyance difference (ΔL) equal to the area between curve B and curve A occurs in the example shown in FIG. 2.

As a result, if both rollers start simultaneously when the conveyance operation starts, the conveyance roller 30 will convey the paper conveyance difference ΔL more by the time both rollers reach the specific speed and are controlled to the same paper feed amount. This reduces the amount of slack in the paper 26, and depending upon the conveyance difference ΔL could result in the elimination of slack.

The conveyance control unit 22 of the printer 2 according to this embodiment of the invention therefore applies control that keeps the slack produced in the initial state substantially constant during each conveyance operation. This control method is described in detail below.

FIG. 3 is a flow chart showing steps in the process executed by the conveyance control unit 22. Control of the conveyance operation is described below with reference to FIG. 3. Note that this control is based on eliminating the conveyance (paper feed) difference when drive starts by delaying the start of conveyance roller 30 operation, and the timing when conveyance roller 30 operation starts is determined based on drive data from the previous conveyance operation that well represents current operating conditions, such as how much roll paper 25 remains.

Methods that use the difference in the rise time of the supply roller 29 and conveyance roller 30 (the difference in the time required to reach the specific speed), the difference in the amount of media conveyed before the supply roller 29 and conveyance roller 30 reach the specific speed, or the duty difference of the motors 27 that drive the supply roller 29 and conveyance roller 30 after the specific speed is reached, as the drive data can be executed.

When a start paper feed command is received from the print control unit 21 for a conveyance operation (step S1), the conveyance control unit 22 first gets the wait time ΔT stored in RAM as described above (step S2). This wait time ΔT is the time that the start of conveyance roller 30 operation is delayed, and is information that is determined after each conveyance operation ends and is stored for the next conveyance operation. More specifically, the wait time ΔT is the value that was determined in the previous conveyance operation, and is determined as described more specifically below.

Note that for the first conveyance operation after the printer 2 turns on a predetermined default value stored in NVRAM is acquired. Alternatively, the value of the wait time ΔT determined in each conveyance operation could be stored in NVRAM and the wait time ΔT acquired therefrom.

The conveyance control unit 22 starts driving the supply roller 29 after this command is received (step S3). More specifically, the conveyance control unit 22 starts the motor 27A and continues control so that the conveyance speed of the supply roller 29 reaches the specific speed target. Note that the conveyance control unit 22 controls driving the supply roller 29 and conveyance roller 30 by means of PID control based on output from the encoders 31A and 31B.

After starting driving the supply roller 29, the conveyance control unit 22 waits for the acquired wait time ΔT to pass (step S4), and then starts driving the conveyance roller 30 (step S5). More specifically, the conveyance control unit 22 starts the motor 27B and continues control so that the conveyance speed of the conveyance roller 30 reaches the specific speed target.

When the supply roller 29 and conveyance roller 30 then reach the specific speed, the conveyance control unit 22 continues controlling driving the rollers at the constant speed (step S6).

When a stop paper feed command is received from the print control unit 21 (step S7), the conveyance control unit 22 applies control to stop driving the supply roller 29 and conveyance roller 30 (step S8). This control may simply quickly reduce the speed of both rollers to zero, but preferably stops both rollers so that the paper feed distance of both rollers in the current conveyance operation is the same. This maintains slack in the paper 26 between the supply roller 29 and conveyance roller 30 when the conveyance operation starts.

When the rollers are stopped and the current conveyance operation ends as described above, the conveyance control unit 22 determines the wait time ΔT of the next conveyance operation from the drive status of the supply roller 29 and conveyance roller 30 in the current conveyance operation, and overwrites the value previously stored in RAM with the new wait time ΔT (step S9).

Because this wait time ΔT is used to eliminate the difference in the paper feed distance resulting from the difference in the behavior of the supply roller 29 and conveyance roller 30 when driving starts, a method that determines the wait time ΔT from the behavior of both rollers when driving starts can be used. More specifically, one method determines the wait time ΔT from the difference in the rise times of the supply roller 29 and conveyance roller 30 as described above.

FIG. 4 describes the wait time ΔT. FIG. 4 (A) is similar to the graph in FIG. 2, and shows the change in speed over time when driving the supply roller 29 and conveyance roller 30 starts simultaneously. The difference in the rise time is ΔT1. More specifically, ΔT1 is the difference in the time required for each roller to reach the specific speed target after driving the roller starts.

FIG. 4 (B) shows the change over time in the conveyance speed of the supply roller 29 and conveyance roller 30 when the printer 2 is controlled as described with reference to FIG. 3. As described above, starting driving the conveyance roller 30 as indicated by curve B is delayed wait time ΔT from the start of driving the supply roller 29 indicated by curve A.

As a result, the amount conveyed by both rollers is substantially the same (the areas of ΔL1 and ΔL2 in the figure are substantially the same) by the time the two rollers both the specific speed Vt target (T3 in the figure), and slack in the paper 26 remains substantially constant during the conveyance operation.

Because the rise time difference ΔT1 and the wait time ΔT are substantially proportional, the proportional gain k1 of ΔT=k1×ΔT1 is experimentally determined, and is stored in NVRAM as the relationship information described above. This method therefore determines the time required for supply roller 29 and conveyance roller 30 to reach the specific speed after driving starts (determines TA and TB in the example in FIG. 4 (B)), calculates ΔT1 from the difference therebetween, and then uses the proportional gain k1, the above relationship information, to determine the wait time ΔT from the relationship ΔT=k1×ΔT1.

Note that the drive data stored in RAM as described above is used to determine the wait time ΔT, and this data is appropriately acquired and stored by the conveyance control unit 22. The conveyance speeds of the supply roller 29 and conveyance roller 30, and the duty of the motors 27 (the current supplied to the motors 27 in this example) are also stored at a specific time interval.

A second method determines the wait time ΔT from the media conveyance difference ΔL while the supply roller 29 and conveyance roller 30 rise to speed.

Because the conveyance difference ΔL and the wait time ΔT are substantially proportional, the proportional gain k2 of ΔT=k2×ΔL is experimentally determined, and is stored in NVRAM as relationship information. This method therefore determines the amount of paper 26 conveyed by the supply roller 29 and the conveyance roller 30 between when driving each roller starts and reaches the specific speed after driving starts (TA shown in FIG. 4 (B)), calculates ΔL from the difference therebetween, and then uses the proportional gain k2, the above relationship information, to determine the wait time ΔT from the relationship ΔT=k2×ΔL.

In the example shown in FIG. 4 (B), the conveyance distance during acceleration of the supply roller 29 is the conveyance distance from time T1 to T3, the conveyance distance during acceleration of the conveyance roller 30 is the conveyance distance from time T2 to T4, and ΔL is calculated from the difference between these amounts.

A third method is described next. This method measures the difference in the behavior of the supply roller 29 and conveyance roller 30 when drive starts based on the duty difference ΔD of the drive motors 27 of the supply roller 29 and conveyance roller 30 after the specific speed is reached. More specifically, the wait time ΔT is determined from the duty difference ΔD.

FIG. 5 shows an example of the change over time in the duty of motors 27A and 27B. The duty expresses the current supplied to the motors 27 as a relative value, and a greater value indicates the force that should be applied to the roller is greater.

FIG. 5 shows the duty of the motor 27A (curve A) and motor 27B (curve B) from the start of driving the supply roller 29 and conveyance roller 30. Because great force is required to start, the duty rises to a peak as shown in FIG. 5, and then settles to a substantially constant duty after the target speed is reached.

That the duty is greater for one of the two rollers to be controlled to the same target speed means that the drive load (the power required to drive the roller) is greater, that is, that the back tension on the supply roller 29 is greater. The delay in the rise of the roller speed when drive starts can therefore be determined from the duty difference. This method therefore determines the wait time ΔT from the duty difference. Because the duty varies greatly and is not stable, the duty difference used at the start of drive control is the duty difference ΔD during the stable period (such as period P in FIG. 5) after the specific speed is reached and becomes stable.

Because the duty difference ΔD and the wait time ΔT are substantially proportional, the proportional gain k3 of ΔT=k3×ΔD is experimentally determined, and is stored in NVRAM as relationship information described above. This method therefore determines the typical duty of each roller after the supply roller 29 and conveyance roller 30 reach the specific speed, calculates ΔD from the difference therebetween, and then uses the proportional gain k3, the above relationship information, to determine the wait time ΔT from the relationship ΔT=k3×ΔD.

Note that this typical duty could be the average of plural duty values detected in a preset time.

The relationship between the information for determining the wait time (ΔT1, ΔL, ΔD, referred to generally as ΔX) and the wait time ΔT is linear in the three methods described above, but this relationship could be expressed as a function f where ΔT=f(ΔX). In this case, function f is determined in advance and stored as relationship information.

When the wait time ΔT is thus determined, stored in RAM, and updated (step S9), the control process for the conveyance operation ends, and the same process thereafter repeats in the next conveyance operation.

Because the relationship between the information for determining the wait time (ΔT1, ΔL, ΔD) and the wait time differs according to the type of paper 26, the relationship information described above could be prepared and stored for different types of paper used in the printer 2.

As described above, the media conveyance system of the printer 2 according to this embodiment of the invention appropriately delays the start time of the conveyance roller 30 based on behavior during the previous conveyance operation, that is, based on the preceding conditions. Conveyance delay when the supply roller 29 starts can therefore be desirably eliminated, and the slack in the paper 26 between the supply roller 29 and conveyance roller 30 can be held constant with substantially no change during the conveyance operation. As a result, even if conveyance conditions change, such as how much of the paper roll 25 remains, back tension on the conveyance roller 30 can be constantly eliminated, the paper 26 can be accurately supplied from the conveyance roller 30 to the printing position, and print quality can be improved.

Because there is substantially no change in the initial amount of slack, the paper 26 will not contact parts along the conveyance path due to excess slack, minimal space is needed for the conveyance path, and increasing conveyance device size to eliminate back tension is not necessary.

The conveyance method also works more effectively in a device that uses paper roll 25, which can easily cause the back tension on the supply roller 29 to change.

Accurate control is also possible because the wait time {circle around (x)}T of the conveyance roller 30 is determined using suitable parameters.

Even more accurate control is also possible by applying control using relationship information prepared for different types of paper.

While the print medium is paper in the embodiment described above, the invention is not so limited and can be used with other types of sheet media.

The invention being thus described, it will be obvious that it may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims. 

1. A conveyance device comprising: an upstream roller that supplies a sheet medium to be processed to a conveyance path; a downstream roller that conveys the supplied medium to a processing position; and a control unit that controls driving the upstream roller and the downstream roller to convey the sheet medium at a constant speed, and determines the starting time of the upstream roller and the downstream roller when starting the conveyance operation based on specific information about the upstream roller and the downstream roller in a previous conveyance operation.
 2. The conveyance device described in claim 1, wherein: the specific information is information about the behavior of the upstream roller and the downstream roller in the previous conveyance operation until reaching the constant speed.
 3. The conveyance device described in claim 1, wherein: the specific information is drive information at the constant speed of the upstream roller and the downstream roller in the previous conveyance operation.
 4. The conveyance device described in claim 1, wherein: determination of the starting time delays the starting time of the downstream roller relative to the upstream roller.
 5. The conveyance device described in claim 2, wherein: the information about the behavior until reaching the constant speed is the time from when the upstream roller and the downstream roller start until the constant speed is reached.
 6. The conveyance device described in claim 2, wherein: the information about the behavior until reaching the constant speed is the amount the sheet medium is respectively conveyed by the upstream roller and the downstream roller from when the upstream roller and the downstream roller start until the time required for the upstream roller to reach the constant speed after starting passes.
 7. The conveyance device described in claim 3, wherein: the drive information at the constant speed is information indicating the amount of power required to drive the upstream roller and the downstream roller.
 8. The conveyance device described in claim 2, wherein: information indicating the relationship between the information about the behavior until reaching the constant speed or the drive information at the constant speed, and the starting time, is stored in advance for different types of sheet media.
 9. The conveyance device described in claim 3, wherein: information indicating the relationship between the drive information at the constant speed and the starting time is stored in advance for different types of sheet media.
 10. The conveyance device described in claim 1, wherein: the sheet medium is supplied from a roll to the upstream roller.
 11. A printing device that comprises the conveyance device described in any of claims 1 to 10, and prints on the sheet medium at the processing position.
 12. A conveyance method of a conveyance device including an upstream roller that supplies a sheet medium to be processed to a conveyance path, a downstream roller that conveys the supplied medium to a processing position, and a control unit that controls driving the upstream roller and the downstream roller to convey the sheet medium at a constant speed, wherein: the control unit determines the starting time of the upstream roller and the downstream roller when starting the conveyance operation based on specific information about the upstream roller and the downstream roller in a previous conveyance operation.
 13. The conveyance method described in claim 12, wherein: the specific information is information about the behavior of the upstream roller and the downstream roller in the previous conveyance operation until reaching the constant speed.
 14. The conveyance method described in claim 12, wherein: the specific information is drive information at the constant speed of the upstream roller and the downstream roller in the previous conveyance operation.
 15. The conveyance method described in claim 12, wherein: determination of the starting time delays the starting time of the downstream roller relative to the upstream roller.
 16. The conveyance method described in claim 13, wherein: the information about the behavior until reaching the constant speed is the time from when the upstream roller and the downstream roller start until the constant speed is reached.
 17. The conveyance method described in claim 13, wherein: the information about the behavior until reaching the constant speed is the amount the sheet medium is respectively conveyed by the upstream roller and the downstream roller from when the upstream roller and the downstream roller start until the time required for the upstream roller to reach the constant speed after starting passes.
 18. The conveyance method described in claim 14, wherein: the drive information at the constant speed is information indicating the amount of power required to drive the upstream roller and the downstream roller. 